pwd                     # Print working directory
ls -la                  # List all files with details
cd /var/log             # Change directory
tree -L 2               # Show directory tree (2 levels)
find / -name "*.log" -mtime -1   # Files modified in last 24h
du -sh /var/*           # Disk usage per directory
df -h                   # Disk space on all mounts

# Search with grep
grep -r "ERROR" /var/log/
grep -c "404" access.log          # Count matches
grep -i "timeout" app.log         # Case-insensitive

# Stream processing with awk
awk '{print $1, $9}' access.log   # Print IP and status code
awk -F: '{print $1}' /etc/passwd  # Custom delimiter

# Transform with sed
sed -i 's/old/new/g' config.txt   # In-place find & replace
sed -n '10,20p' logfile.txt       # Print lines 10-20

# Sort and deduplicate
sort access.log | uniq -c | sort -rn | head -20

ps aux                          # All running processes
ps aux | grep nginx             # Find specific process
top                             # Real-time process viewer
htop                            # Better top (if installed)
kill -15 <pid>                  # Graceful shutdown
kill -9 <pid>                   # Force kill
systemctl status nginx          # Service status (systemd)
systemctl restart nginx         # Restart service
journalctl -u nginx -f          # Follow service logs

ip addr show                    # Show IP addresses
ss -tulnp                       # Show listening ports
curl -I https://example.com     # HTTP headers only
curl -v https://example.com     # Verbose HTTP request
wget -q -O - https://example.com/health   # Simple health check
dig example.com                 # DNS lookup
nslookup example.com            # Alternative DNS lookup
traceroute example.com          # Network path tracing
netstat -rn                     # Routing table

chmod 755 script.sh             # rwxr-xr-x
chmod +x deploy.sh              # Add execute permission
chown -R www-data:www-data /var/www  # Change ownership
whoami                          # Current user
id                              # User and group info
sudo -u postgres psql           # Run command as another user

tar -czf backup.tar.gz /data    # Create compressed archive
tar -xzf backup.tar.gz          # Extract archive
tar -tzf backup.tar.gz          # List contents
zip -r archive.zip folder/      # Create zip

uname -a                        # Kernel info
cat /etc/os-release             # OS details
free -h                         # Memory usage
uptime                          # System uptime and load
lscpu                           # CPU information

# Watch a log file in real-time
tail -f /var/log/syslog

# Find the 10 largest files
find / -type f -exec du -h {} + 2>/dev/null | sort -rh | head -10

# Check which process is using a port
lsof -i :8080

# Test if a port is open
nc -zv hostname 443

# Base64 encode/decode (useful for K8s secrets)
echo -n "mypassword" | base64
echo "bXlwYXNzd29yZA==" | base64 -d